The invention relates generally to devices for delivering oxygen to cells and more specifically to devices which deliver oxygen in situ to cells in the body of an organism.
Techniques to transplant cells into people in need of the functions that these cells provide have application in the treatment of a variety of chronic conditions including diabetes, hemophilia, dwarfism, anemia, kidney failure, hepatic failure, familial hypercholesterolemia, immunodeficiency disorders, pituitary disorders, and central nervous system disorders.
Cell implantation techniques are typically limited by shortages of cells. For example, successful transplantation of insulin-secreting cells or tissue into people with diabetes has been a challenge because of the obvious shortage of human islet tissue. The approximately 3000 cadaver pancreases that could be available each year in the U.S.A. come nowhere near to meeting the needs of people with Insulin Dependent Diabetes Mellitus (IDDM). Use of cells/tissue from other species as xenogeneic cells may therefore provide the fastest path to clinical application.
Immunobarrier devices have been developed as a means of protecting xenogeneic cells from transplantation rejection by a host organism. The xenogeneic cells are encapsulated at a high, tissue-like density or are dispersed in the form of individual cells or cell aggregates (e.g., islets of Langerhans) in an extracellular gel matrix such as agar, alginate, or chitosan, within these devices.
High-density culture, if attainable, is advantageous because it minimizes the size of the implanted device used in a particular application. This is desirable because the complexity and difficulty of the application increases with the volume of implanted cells/tissue. Consequently, applications which have tended to require the least amount of transplanted cells/tissue, such as central nervous system applications, have been the first to advance to clinical testing.
Maintenance of the viability and function of implanted cells within an immunobarrier device is essential and limited by the supply of nutrients and oxygen which can be provided to the cells from the host. Apoptosis in transplanted tissues has been observed and may be a general response to severe hypoxia, as well as to methods of isolation and culture, glycemic state, and the nonspecific inflammatory reaction associated with the transplantation procedure.
The impact of hypoxia is also influenced by the type of cells/tissues being implanted. For example, pancreatic islet cells are especially prone to oxygen supply limitations because they have a relatively high oxygen consumption rate. They are normally highly vascularized and are supplied blood at arterial pO2. When cultured in vitro under normoxic conditions, islets develop a necrotic core, the size of which increases with increasing islet size, as is to be expected as a result of oxygen diffusion and consumption within the islet. However, the death of implanted cells due to hypoxia is not the only concern. Oxygen levels high enough to keep cells alive can nonetheless have deleterious effects on cell functions that require higher cellular ATP concentrations, for example, ATP-dependent insulin secretion.
Only scant attention has been paid to the issue of islet viability within implanted immunobarrier devices. However, recently, critical parameters such as the number and volume of viable islet cells that can be supported by such devices, and the development of islet necrosis and fibrosis in such devices has begun to be examined. It is clear from these recent studies that all attempts to support larger volumes of islet tissue in high-density culture (i.e., where all or most of the internal device volume is occupied by viable islet tissue) have led to massive islet necrosis, invariably in regions furthest from the oxygen source. As with transplantation of naked cells, the hypoxic environment for several days following transplantation appears to be a critical problem. For example, most of the loss of viable xcex2-cell mass undoubtedly occurs during the first few days after transplantation within these devices.
With few exceptions, only by suspending islets in an extracellular gel matrix at very low islet volume fractions (e.g., 1 to 5%), which greatly increases the size of the implanted device, have investigators been able to maintain the viability of the initially loaded islets. However, use of such low tissue density puts undesirable constraints on the maximum number of islets that can be supported in a device of a size suitable for surgical implantation.
Attempts to modify the design of immunobarrier devices have been made to try to overcome these limitations. A biohybrid artificial pancreas for insulin secretion known in the art consists of a semipermeable membrane tube through which arterial blood flows. The membrane tube is surrounded by the implanted tissue which is, in turn, contained in a housing. This approach provides the highest available pO2 (100 mm Hg) but suffers from the need to open the cardiovascular system; thus, it may be limited to only a small fraction of patients.
One alternative is an extravascular device in the form of a planar or cylindrical diffusion chamber implanted, for example, in subcutaneous tissue or intraperitoneally. Such devices are exposed to the mean pO2 of the microvasculature (about 40 mm Hg) limiting the steady state thickness of viable tissue that can be supported. Further limits are imposed when such devices are implanted into soft tissue. If a foreign body response occurs, an avascular fibrotic tissue layer adjacent to the chamber can be produced, typically on the order of 100 xcexcm thick. This fibrotic tissue increases the distance between blood vessels and implant, and the fibroblasts in fibrotic tissue layer also consume oxygen. Oxygen deficits are especially likely during the first few days after implantation before neovascularization has a chance to occur. Anoxia may exist within regions of the device, leading to death of a substantial fraction of the initially implanted tissue.
Microporous membranes that induce neovascularization at the device-host tissue interface have also been used. This angiogenic process takes 2-3 weeks for completion, and the vascular structures induced remain indefinitely. By bringing some blood vessels close to the implant, oxygen delivery is improved. Oxygen delivery also may be improved by prevascularizing the device, e.g, by infusion of an angiogenic factor(s) through the membranes into the surrounding tissue.
Another means of implanting cells in an extravascular environment involves the use of spherical microcapsules. The microcapsules comprise small quantities of cells enclosed in a semipermeable membrane and can be implanted in an extravascular space, for example, in the peritoneal space. However, the large volume of microcapsules employed, and the tendency for most to permanently attach to peritoneal surfaces, may lead to clinical problems. Thus, despite encouraging results with various tissues and applications, the problem of oxygen transport limitations remain.
The present invention improves the viability and function of encapsulated tissue.
The invention provides an oxygen generator device for delivering oxygen to cells or to a cell compatible fluid. The oxygen generator device disclosed herein has application for in vitro or in vivo use. In one aspect the device is placed in proximity to a cell compatible fluid. In another aspect, the oxygen generator is placed in proximity to cells for which supplemental oxygen is desired. In a further aspect of the invention, the oxygen generator is placed in proximity to a cell encapsulating device. The oxygen generator disclosed herein provides a system to deliver oxygen to cells in situ in the body of an organism.
In one embodiment of the invention, the oxygen generator is an electrolyzer device which electrolyzes water into oxygen and hydrogen. In another embodiment of the invention, the oxygen generator is in the form of a thin, multilayer electrolyzer sheet and is permeable to gas and water vapor but impermeable to liquids and dissolved material. In a further embodiment of the invention, the oxygen generator comprises a multilayer electrolyzer sheet having a proton exchange membrane sandwiched by an anode layer and a cathode layer. In a further embodiment of the invention, the device comprises a multilayer electolyzer sheet adapted for mating to a container containing cells.
In one embodiment of the invention, the oxygen generator is in communication with an energy source, such as a battery. In another embodiment of the invention, the battery is rechargeable transcutaneously. In a further embodiment of the invention, the battery is recharged using a transcutaneous energy transfer system (TET) system. In a further embodiment, the invention may be operated directly and continuously from a battery-powered TET system.
In one embodiment of the invention, the oxygen generator is provided in proximity to cells in vitro. The oxygen generator is provided, either within a container containing the cells, or as an integral part of the container. In one embodiment of the invention, the oxygen generator is provided, either within, or as an integral part of, a cell-containing cartridge in an extracorporeal circuit device.
In a further embodiment of the invention, the oxygen generator is provided as an in-line oxygenator through which blood, plasma, and other bodily fluids may flow. In this embodiment, the oxygen generator is mated to, contained within, or is an integral part of, a hollow tube through which blood, plasma, and other bodily fluids, or culture medium may flow.
In one embodiment, the invention provides a system for delivering oxygen in situ to cells within the body of an organism. In this embodiment of the invention, the system comprises an oxygen generator positioned in proximity to a cell encapsulating chamber and is implanted within the body of an organism. The cell encapsulating chamber comprises a containment space for cells bounded by a semipermeable barrier layer which acts as a selective diffusion layer, allowing selected components to enter and leave the cell encapsulating chamber.
In a further embodiment of the invention, the system includes an oxygen generator which comprises a multilayer electrolyzer sheet mated to a cell encapsulating chamber comprising two semipermeable membranes sealed together by a ring seal.
In another embodiment of the invention, the system for delivering oxygen in situ comprises a cell encapsulating chamber which defines an immunoisolation chamber. In this embodiment, the semipermeable barrier layer of the immunoisolation chamber immunoisolates cells contained within the chamber when the device is exposed to components of the immune system. In a further embodiment, the invention relates to a system implantable in the body of an organism for growing tissue in immunoisolation while providing supplemental oxygen to the tissue.
In another embodiment of the invention, the system for delivering oxygen in situ comprises an oxygen generator provided in proximity to, or mated with, a semipermeable membrane tube through which blood, plasma, and other bodily fluids may flow. In another embodiment of the invention, the membrane tube is surrounded by implanted tissue. In a further embodiment of the invention, the membrane tube and tissue is contained within a housing.
The invention also relates to a system for delivering oxygen in situ to cells which are not contained within a cell encapsulating device. In a one embodiment of the invention, the system for delivering oxygen in situ comprises an oxygen generator placed in proximity to cell-containing microcapsules which are free to migrate within an intraperitoneal space. In another embodiment of the invention, the system comprises an oxygen generator positioned in proximity to a cell-supporting, biocompatible, polymeric scaffold within the body of an organism. In this embodiment, the oxygen generator can be used to maintain, and support the growth of artificial tissues.
In a further embodiment of the invention, two oxygen generators are placed back to back with cells on both sides of the oxygen generator, maximizing the amount of oxygen that can be delivered to cells.
In another embodiment, the system is designed to deliver oxygen in situ to cells located at a distance from the oxygen generator. A tube with low oxygen permeability is attached to the oxygen generator. Generated oxygen is transferred through the tube to a flexible oxygen distributor fabricated from oxygen-permeable membranes. In one aspect of the invention, the oxygen distributor is placed in proximity to cells, tissues, or organs, for which supplemental oxygen is desired. In another aspect of the invention, the oxygen distributor is placed in proximity to a cell encapsulating device. The flexible oxygen distributor provides a means to deliver oxygen from an oxygen generator located at a distance to cells, tissues, or organs located anywhere, and having any shape, within the body of an organism.
In a further embodiment of the invention, the oxygen generator is an oxygen transfer device which electrochemically transfers oxygen from the cathode side to the anode side of the oxygen generator substantially without the generation of hydrogen.
The invention also relates to methods of delivering oxygen to cells in vitro comprising positioning an oxygen generator in proximity to the cells. In one embodiment of the invention, the method comprises delivering oxygen to cells contained within a container, such as a cell culture dish or a flask. In another embodiment of the invention, the method comprises placing an oxygen generator in proximity to the cells within a cell-containing cartridge in an extracorporeal circuit or in culture medium in a perfusion circuit. In another embodiment of the device, the oxygen generator is provided as an in-line oxygenator through which culture medium flows. In this embodiment, oxygenated culture medium flows through the cell-containing cartridge and is then discarded or recycled through the oxygenator for reoxygenation. In a further embodiment of the invention, the oxygenated culture medium is made to flow in and around an organ for transplantation, such as heart, kidney, liver, or pancreas, during the period when it is stored or shipped.
The invention also relates to a method of delivering oxygen in situ to cells within a body of an organism. In this embodiment of the invention, an oxygen generator is placed in proximity to cells which are free to migrate within an intraperitoneal space. In another embodiment of the invention, the method includes implanting a system within the body of an organism, the system comprising an oxygen generator which is in proximity to a cell encapsulating chamber, such as an immunoisolation chamber.